ETIC MR1200, User Manual

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Operation
Page 30 ETIC - MR1200 - USER MANUAL - 908709-02
4.2.2. Format and coding
The MR1200 assembles the data sent by its control terminal into packets of varying size depending on
the amount of data.
The packets are themselves sub-divided into 64 bit frames, of which 16 bits are used for the cyclic
redundancy check code (CRC) when in « base mode ».
Before being transmitted each packet is preceded by a sequence of bits whose job is to synchronise the
two modems (SYNC).
4.2.3. Operating conditions
radio channel access
At rest, the MR1200 permanently checks for activity on the radio channel ; a transmit command may
simply be refused if the channel is occupied (absence of carrier for at least 0.1 seconds).
Half duplex timing
When transmission begins the MR1200 systematically delays sending the first data in order to allow
transceiver RF stages to develop full power (
tp_alt parameter).
Data packet size
Reducing or increasing data packet size will force the communicating modems to re-synchronise, the
frequency depending on the application and the quality of the radio link (
f_paq parameter).
Half duplex transmission
The MR1200 control of T/R switching (half duplex) is transparent to the terminal. The use of buffers
enables terminal-modem dialogue to remain duplex throughout.
The modem will switch from transmission to reception according to a configurable timing parameter
(
tp_ae parameter), when there is no more data to be sent.
The MR1200 stays in the receive state while data is incoming, reverting to transmission at the end of a
second configurable time delay if the terminal has new data (
tp_ar parameter).
Received data restoration and error treatment (base mode)
The CRC of each frame received is tested, and frames with CRC errors are either eliminated or
signalled using a configurable character (
error parameter). The MR1200 extracts the data from each
error-free frame, placing the bytes end to end.
The data is re-transmitted to the terminal either as it arrives, or as a block of data once the last frame
has been received and restored (" block mode " option,
bloc parameter).